Synthesis and Characterization of 2D Molybdenum Carbide (MXene)

Large scale synthesis and delamination of 2D Mo₂CT _x (where T is a surface termination group) has been achieved by selectively etching gallium from the recently discovered nanolaminated, ternary transition metal carbide Mo₂Ga₂C. Different synthesis and delamination routes result in different flake morphologies. The resistivity of free‐standing Mo₂CT _x films increases by an order of magnitude as the temperature is reduced from 300 to 10 K, suggesting semiconductor‐like behavior of this MXene, in contrast to Ti₃C₂T _x which exhibits metallic behavior. At 10 K, the magnetoresistance is positive. Additionally, changes in electronic transport are observed upon annealing of the films. When 2 μm thick films are tested as electrodes in supercapacitors, capacitances as high as 700 F cm^?3 in a 1 m sulfuric acid electrolyte and high capacity retention for at least 10,000 cycles at 10 A g^?1 are obtained. Free‐standing Mo₂CT _x films, with ≈8 wt% carbon nanotubes, perform well when tested as an electrode material for Li‐ions, especially at high rates. At 20 and 131 C cycling rates, stable reversible capacities of 250 and 76 mAh g^?1, respectively, are achieved for over 1000 cycles.     

Layered Platinum Dichalcogenides (PtS2, PtSe2, and PtTe2) Electrocatalysis: Monotonic Dependence on the Chalcogen Size

Presently, research in layered transition metal dichalcogenides (TMDs) for numerous electrochemical applications have largely focused on Group 6 TMDs, especially MoS₂ and WS₂, whereas TMDs belonging to other groups are relatively unexplored. This work unravels the electrochemistry of Group 10 TMDs: specifically PtS₂, PtSe₂, and PtTe₂. Here, the inherent electroactivities of these Pt dichalcogenides and the effectiveness of electrochemical activation on their charge transfer and electrocatalytic properties are thoroughly examined. By performing density functional theory (DFT) calculations, the electrochemical and electrocatalytic behaviors of the Pt dichalcogenides are elucidated. The charge transfer and electrocatalytic attributes of the Pt dichalcogenides are strongly associated with their electronic structures. In terms of charge transfer, electrochemical activation has been successful for all Pt dichalcogenides as evident in the faster heterogeneous electron transfer (HET) rates observed in electrochemically reduced Pt dichalcogenides. Interestingly, the hydrogen evolution reaction (HER) performance of the Pt dichalcogenides adheres to a trend of PtTe₂ > PtSe₂ > PtS₂ whereby the HER catalytic property increases down the chalcogen group. Importantly, the DFT study shows this correlation to their electronic property in which PtS₂ is semiconducting, PtSe₂is semimetallic, and PtTe₂ is metallic. Furthermore, Pt dichalcogenides are effectively activated for HER. Distinct electronic structures of Pt dichalcogenides account for their different responses to electrochemical activation. Among all activated Pt dichalcogenides, PtS₂ shows most accentuated improvement as a HER electrocatalyst with an exceptional 50% decline in HER overpotential. Knowledge on Pt dichalcogenides provides valuable insights in the field of TMD electrochemistry, in particular, for the currently underrepresented Group 10 TMDs.     

MoSe2 Enhanced Raindrop Triboelectric Nanogenerators and Its Energy Conversion Analysis

Triboelectric nanogenerators (TENGs) are considered one of the most effective methods for harvesting irregular and low-frequency raindrop energy. In this work, molybdenum selenide (MoSe₂) nanosheets act as intermediate layers for improving droplet-based TENG performance. Consequently, without surface etching process, the short-circuit current (I_sc) and open-circuit voltage (V_oc) of the TENG can reach as high as 1.2 mA and 120 V, respectively. Furthermore, precise energy analysis based on an optimization model for input energy calculation is carried out, allowing conversion efficiency to be calculated under diverse conditions. Finally, an all-solid supercapacitor is fabricated for integration with the TENG. An intelligent wireless sensing system, powered by the integrated TENG and capacitor, is demonstrated for monitoring environmental information. This study provides new insights into intermediate-layer materials' selection and action mechanisms. It fills a gap in the research on a precise model of theoretical energy conversion efficiency calculation. The integrated devices and sensing applications will provide strategies for creating smart cities.     

Chemical Vapor Deposition Syntheses of Wafer-Scale 2D Transition Metal Dichalcogenide Films toward Next-Generation Integrated Circuits Related Applications

2D semiconducting transition metal dichalcogenides (TMDCs), most with a formula of MX₂ (M=Mo, W; X=S, Se, etc.), have emerged as promising channel materials for next-generation integrated circuits, considering their dangling-bond-free surfaces, moderate bandgaps, relatively high carrier mobilities, etc. Wafer-scale preparation of 2D MX₂ films holds fundamental significance for realizing their applications. Chemical vapor deposition (CVD) is recognized as the most promising method for preparing electronic-grade 2D MX₂ films. This review hereby summarizes the recent progress in CVD syntheses of wafer-scale 2D MX₂ films and their applications in logic operations, data storage, and image capturing/processing related fields. The first part focuses on the wafer-scale syntheses of 2D MX₂ films through designing homogeneous metal precursor supply routes (e.g., precoating soluble precursor, feeding gaseous precursor, designing independent multisource supply or face-to-face metal precursor supply routes). The second part highlights the epitaxial growth of monolayer MX₂ single crystals on single-crystal Au substrates and well-designed sapphire substrates. The third part introduces various functional device/circuit related applications of CVD-derived 2D MX₂ wafers. Finally, challenges and prospects are discussed from the viewpoints of the controlled synthesis, reliable characterization, and damage-free transfer of 2D MX₂, as well as the fabrication and integration of high-performance devices.     

2D Rhenium Dichalcogenides: From Fundamental Properties to Recent Advances in Photodetector Technology

Van der Waals (vdW) materials of transition metal dichalcogenides (TMD) family with semiconducting properties are currently at the forefront of research in the field of optoelectronics. The ability to couple them with one another at atomic interface precision in a synergistic way opens up unprecedented opportunities to design photodetectors of broad spectral range with excellent figures of merits not accessible to discrete materials. Recent years have seen a surge of interest in group VII TMD materials (ReS₂ and ReSe₂) due to their strong optical response from bulk to monolayer and good ambient stability. Their band gap energies spanning over visible and near-infrared ranges and the strong linear polarization sensitivity stemming from the distorted octahedral symmetry, are ideally suited for polarization-sensitive photodetectors. This review aims at providing a comprehensive understanding of the fundamental properties, optical identification of various structural features, long-debated question of band gap nature and interlayer coupling, and recent advances in the development of photodetectors based on ReS₂, ReSe₂, and their vdW heterostructures with other layered materials of practical importance. We critically review various conceptual device designs implemented based on band engineering, emphasize on the merits of these photodetectors and their potential applications, and provide an outlook for future prospects.     

Controlled Synthesis of 2D Palladium Diselenide for Sensitive Photodetector Applications

Palladium diselenide (PdSe₂), a thus far scarcely studied group‐10 transition metal dichalcogenide has exhibited promising potential in future optoelectronic and electronic devices due to unique structures and electrical properties. Here, the controllable synthesis of wafer‐scale and homogeneous 2D PdSe₂ film is reported by a simple selenization approach. By choosing different thickness of precursor Pd layer, 2D PdSe₂ with thickness of 1.2–20 nm can be readily synthesized. Interestingly, with the increase in thickness, obvious redshift in wavenumber is revealed by Raman spectroscopy. Moreover, in accordance with density functional theory (DFT) calculation, optical absorption and ultraviolet photoemission spectroscopy (UPS) analyses confirm that the PdSe₂ exhibits an evolution from a semiconductor (monolayer) to semimetal (bulk). Further combination of the PdSe₂ layer with Si leads to a highly sensitive, fast, and broadband photodetector with a high responsivity (300.2 mA W^?1) and specific detectivity (≈10¹³ Jones). By decorating the device with black phosphorus quantum dots, the device performance can be further optimized. These results suggest the as‐selenized PdSe₂ is a promising material for optoelectronic application.     

Atomically Thin MoS2: A Versatile Nongraphene 2D Material

Two‐dimensional inorganic materials are emerging as a premiere class of materials for fabricating modern electronic devices. The interest in 2D layered transition metal dichalcogenides is especially high. Particularly, 2D MoS₂ is being heavily researched due to its novel functionalities and its suitability for a wide range of electronic and optoelectronic applications. In this article, the progress in mono/few layer(s) MoS₂ research is reviewed by focusing primarily on the layer dependent evolution of crystal, phonon, and electronic structure. The review includes extensive detail into the methodologies adapted for single or few layer(s) MoS₂ growth. Further, the review covers the versatility of 2D MoS₂ for a broad range of device applications. Recent advancements in the field of van der Waals heterostructures are also highlighted at the end of the review.     

Sulfur‐Mastery: Precise Synthesis of 2D Transition Metal Dichalcogenides

Chemical vapor deposition (CVD) has been developed as the most promising method for the growth of transition metal dichalcogenides (TMDs). In this work, the key factor determining the growth of TMDs is ascertained. A straightforward method is devised to directly achieve a holistic control of thickness, shape, and size of WS₂ flakes via a single parameter control, namely, the status of the S‐precursor. The thickness‐dependent growth of WS₂ flakes from mono‐ to quad‐layers is achieved by precise control of the feeding rate of elemental S‐precursor. Moreover, the explicit control over amount and exposure time of S‐precursor determines the most optimum combination of these parameters to tune the shape of the crystals from triangular to hexagonal with appropriate size. Hence, the experimental findings provide a promising strategy to engineer the growth evolution of WS₂ atomic layers by fine tuning of the sulfur supply, paving a pathway to scalable electronic and photonic devices.     

Atomic Insight into Thermolysis‐Driven Growth of 2D MoS2

Understanding and controlling the transformations of transition metal dichalcogenides (TMDs) from amorphous precursors into two‐dimensional (2D) materials is important for guiding synthesis, directing fabrication, and tailoring functional properties. Here, the combined effects of thermal energy and electron beam irradiation are explored on the structural evolution of 2D MoS₂ flakes through the thermal decomposition of a (NH₄)₂MoS₄ precursor inside an ultrahigh vacuum (10^?9 Torr) scanning transmission electron microscope (STEM). The influence of reaction temperature, growth substrate, and the initial precursor morphology on the resulting 2D MoS₂ flake morphology, edge structures, and point defects are explored. Although thermal decomposition occurs extremely fast at elevated temperatures and is difficult to capture using current STEM techniques, electron beam irradiation can induce local transformations at lower temperatures, enabling direct observation and interpretation of critical growth steps including oriented attachment and transition from single‐ to multilayer structures at atomic resolution. An increase in the number of layers of the MoS₂ flakes from island growth is investigated using electron beam irradiation. These findings provide insight into the growth mechanisms and factors that control the synthesis of few‐layer MoS₂ flakes through thermolysis and toward the prospect of atomically precise control and growth of 2D TMDs.     

Periodical Ripening for MOCVD Growth of Large 2D Transition Metal Dichalcogenide Domains

2D semiconductors, especially 2D transition metal dichalcogenides (TMDCs), have attracted ever-growing attention toward extending Moore's law beyond silicon. Metal–organic chemical vapor deposition (MOCVD) has been widely considered as a scalable technique to achieve wafer-scale TMDC films for applications. However, current MOCVD process usually suffers from small domain size with only hundreds of nanometers, in which dense grain boundary defects degrade the crystalline quality of the films. Here, a periodical varying-temperature ripening (PVTR) process is demonstrated to grow wafer-scale high crystalline TMDC films by MOCVD. It is found that the high-temperature ripening significantly reduces the nucleation density and therefore enables single-crystal domain size over 20 µm. In this process, no additives or etchants are involved, which facilitates low impurity concentration in the grown films. Atom-resolved electron microscopy imaging, variable temperature photoluminescence (PL) spectroscopy, and electrical transport results further confirm comparable crystalline quality to those observed in mechanically exfoliated TMDC films. The research provides a scalable route to produce high-quality 2D semiconducting films for applications in electronics and optoelectronics.     

Revealing the Atomic Defects of WS2 Governing Its Distinct Optical Emissions

Defects and their spatial distribution are crucial factors in controlling the electronic and optical properties of semiconductors. By using scanning transmission electron microscopy and electron energy loss spectroscopy, the type of impurities/defects in WS₂ subdomains with different optical properties is successfully assigned. A higher population of Cr impurities is found in the W‐terminated edge domain, while the S‐terminated domain contains more Fe impurities, in accordance with the luminescence characteristics of chemical‐vapor‐grown WS₂ of a hexagonal shape. In agreement with the first‐principles calculations, the domains with Cr substitutional dopants exhibit strong trion emission. Fe atoms tend to gather into trimer configuration and introduce deep acceptor levels which compensate the n‐type doping and suppress trion emission. It is also discovered that the domain with higher luminescence but smaller defect concentration tends to get oxidized more rapidly and degrade the 2D structure with many triangular holes. Excitons tend to accumulate at the edges of the oxidized triangular holes and results in enhanced PL emission. The findings indicate that choosing stable elements as dopant and controlling the number of specific edge structures within a crystal domain of 2D transitional metal dichalcogenides can be a new route to improve the optical properties of these materials.     

Photoluminescence from Liquid‐Exfoliated WS2 Monomers in Poly(Vinyl Alcohol) Polymer Composites

While liquid phase exfoliation can be used to produce nanosheets stabilized in polymer solutions, very little is known about the resultant nanosheet size, thickness, or monolayer content. The present study uses semiquantitative spectroscopic metrics based on extinction, Raman, and photoluminescence (PL) spectroscopy to investigate these parameters for WS₂ nanosheets exfoliated in aqueous poly(vinyl alcohol) (PVA) solutions. By measuring Raman and PL simultaneously, the monolayer content can be tracked via the PL/Raman intensity ratio while varying processing conditions. The PL is found to be maximized for a stabilizing polymer concentration of 2 g L^?1. In addition, the monolayer content can be controlled via the centrifugation conditions, exceeding 5% by mass in some cases. These techniques have allowed tracking the ratio of PL/Raman in a droplet of polymer‐stabilized WS₂ nanosheets as the water evaporates during composite formation. No evidence of nanosheet aggregation is found under these conditions although the PL becomes dominated by trion emission as drying proceeds and the balance of doping from PVA/water changes. Finally, bulk PVA/WS₂ composites are produced by freeze drying where >50% of the monolayers remain unaggregated, even at WS₂ volume fractions as high as 10%.     

Probing Functional Structures,Defects, and Interfaces of 2D Transition Metal Dichalcogenides by Electron Microscopy

2D transition metal dichalcogenides (TMDs) exhibit remarkable properties that are strongly influenced by their atomic structures, as well as by various types of defects and interfaces that can be precisely engineered and controlled. These features make 2D TMDs and TMD-based materials highly promising for a wide range of applications in electronics, optoelectronics, magnetism, spintronics, catalysis, energy, etc. By providing a comprehensive approach to understand the structure–property–functionality relationship in materials at the atomic scale, electron microscopy, and spectroscopy techniques have emerged as invaluable tools for studying both the static characteristics and dynamic evolutions of 2D TMDs. In this review, the primary focus lies in exploring intrinsic and artificial structures in TMDs and their heterostructures, along with their corresponding properties, through cutting-edge aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Additionally, recent advancements in the field of in situ visualization and manipulation of 2D TMDs using electron beams are highlighted. It is anticipated that the up-to-date overview provided, along with a glimpse into the future development of STEM-based techniques, will make a substantial contribution to advancing research on 2D materials.     

Misfit Layer Compounds: A Platform for Heavily Doped 2D Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) display a rich variety of instabilities such as spin and charge orders, Ising superconductivity, and topological properties. Their physical properties can be controlled by doping in electric double-layer field-effect transistors (FET). However, for the case of single layer NbSe₂, FET doping is limited to ≈ 1 × 10¹⁴ cm⁻², while a somewhat larger charge injection can be obtained via deposition of K atoms. Here, by performing angle-resolved photoemission spectroscopy, scanning tunneling microscopy, quasiparticle interference measurements, and first-principles calculations it is shown that a misfit compound formed by sandwiching NbSe₂ and LaSe layers behaves as a NbSe₂ single layer with a rigid doping of 0.55–0.6 electrons per Nb atom or ≈ 6 × 10¹⁴ cm⁻². Due to this huge doping, the 3 × 3 charge density wave is replaced by a 2 × 2 order with very short coherence length. As a tremendous number of different misfit compounds can be obtained by sandwiching TMDs layers with rock salt or other layers, this work paves the way to the exploration of heavily doped 2D TMDs over an unprecedented wide range of doping.     

Experimental Evidence of Anisotropic and Stable Charged Excitons (Trions) in Atomically Thin 2D ReS2

Experimentally observed, stable trions with large binding energy (≈25 meV) in atomically thin monolayer 2D transition metal dichalcogenides MX₂ (M = Mo, W, X = S, Se, and Te) with an isotropic crystal structure have been extensively studied. In contrast, the characteristics of trions in atomically thin 2D materials with an anisotropic crystal structure are not completely understood. Low‐temperature photoluminescence (PL) spectroscopy in few‐layer ReS₂ with an anisotropic crystal structure by applying a gate voltage is described. A new PL peak that emerges below the lower‐energy side of neutral excitons obtained by tuning the gate voltages is attributed to emission from negative trions. Furthermore, the trion binding energy that is strongly dependent on the layer thickness reaches a large value of ≈60 meV in 1L–ReS₂, which is ≈2 times larger than that in other isotropic 2D materials (MX₂). The enhancement of the binding energy reflects the quasi‐1D nature of the trions in anisotropic atomically thin ReS₂. These experimental observations will promote a better understanding of the optical response and applications in new categories of the anisotropic atomically thin 2D materials with a quasi‐1D nature.     

Role of Molecular Sieves in the CVD Synthesis of Large‐Area 2D MoTe2

The synthesis of high‐quality 2D MoTe₂ with a desired phase on SiO₂/Si substrate is crucial to its diverse applications. A side reaction of Te with the substrate Si leading to SiTe and Si₂Te₃ tends to happen during growth, resulting in the failure to obtain MoTe₂. It has been found that molecular sieves can adsorb the silicon telluride byproducts and eliminate the influence of the side reaction during the chemical vapor deposition synthesis of MoTe₂. With the help of molecular sieves, few‐layer 1T′ MoTe₂ can be grown from the MoO_x precursor. Pure 1T′ MoTe₂ and 2H MoTe₂ regions in centimeter‐sized areas synthesized on the same piece of SiO₂/Si substrate can be obtained by using an overlapped geometry. The strategy provides a new method to controllably synthesize MoTe₂ with desired phases and can be generalizable to the synthesis of other tellurium‐based layered materials.